A via hole structure is used to provide contact between a surface electrode and the back surface of a semiconductor substrate and is generally formed by first forming an opening (or recess) by etching from the back surface of the semiconductor substrate and then coating the inside of the opening with a metal layer. This structure eliminates the need for a wire bonding structure, making it possible to reduce the source inductance and hence improve the performance (high frequency characteristics) of the device.
The primary purpose of the via hole structure is to provide contact with a source electrode on the surface or an extension metal of the source electrode. Therefore, approximately a few micros of metal is formed along the inner walls of the via hole to ensure contact. Via hole structures are typically single holes having a uniform diameter. It should be noted that via holes need not necessarily be filled in with metal.
Prior art includes Japanese Laid-Open Patent Publication Nos. S63-155673, H06-326064, 2009-290098, and 2009-33097.
The heat dissipation characteristics of a semiconductor device may affect its performance. Specifically, for example, there are many high output power applications for semiconductor devices having a via hole structure, such as HEMTs and MMICs. In these applications, the upper limit of the temperature of the semiconductor channel primarily determines the maximum output power value of the device, making it important to improve the heat dissipation characteristics of the device in order to increase the maximum output power value.
As described above, since the primary purpose of the via hole structure is to provide contact with a source electrode on the surface, it is common to merely form a metal film along the inner walls of the via hole, instead of filling in the via hole with metal. However, the via hole structure may be filled in with metal in order to utilize the high thermal conductivity of the metal. This means that the heat dissipation characteristics of the device may be improved by filling the via hole with metal.
However, in a via hole structure consisting of one uniform diameter via hole, the via hole is made to extend vertically to an electrode on the surface, meaning that the area of the opening of the via hole is determined by the area of the electrode. That is, in the case of a via hole structure consisting of one uniform diameter via hole, the area of the opening of the via hole cannot exceed the area of the electrode on the surface. This limits the amount of metal that can be accommodated by the via hole structure consisting of one uniform diameter via hole. As a result, the metal filling the via hole structure has only a limited effect in improving the heat dissipation characteristics of the device.
One way to overcome this problem may be to form a via hole structure consisting of a multilevel recess including a first level recess for contact and a second level recess having a greater diameter than the first level recess (that is, to form a via hole structure having different diameter sections). An increased amount of metal can be buried in this via hole structure without disturbing the contact between the structure and the electrode on the surface. Thus this via hole structure having a plurality of different diameter sections has improved heat dissipation characteristics.
In this connection, the above Japanese Laid-Open Patent Publication No. S63-155573 describes a via hole structure with a stepped cross-sectional configuration formed in a semiconductor substrate and filled with metal. Further, the above Japanese Laid-Open Patent Publication No. H06-326064 describes a technique of forming a via hole having a cross-sectional shape including a plurality of steps in order to ensure adequate step coverage, although this via hole is not filled in with metal. However, these publications make no mention of improvement of heat dissipation characteristics.
When a via hole structure consisting of a plurality of recesses (or sections) including a first level recess for contact and a second level recess having a greater diameter than the first level recess is to be formed, it is necessary to solve the problem of how to form the multiple steps. For example, if such a plurality of steps are formed by simply repeating individual etch processes, the number of processes and hence the manufacturing time increase proportionally to the number of steps. In view of the manufacture of actual products, there is a need to provide a method of manufacturing a via hole structure having a plurality of different diameter sections as described above while suppressing an increase in the number of processes and in the processing time.
The present invention has been made to solve the above problems. It is, therefore, an object of the present invention to provide a via hole manufacturing method for manufacturing a via hole structure which is filled with an increased amount of filling material, thereby improving the heat dissipation characteristics, while suppressing an increase in the number of processes or in the processing time. Another object of the present invention is to provide a method of manufacturing a semiconductor device having such via holes.